Please try to always express yourself as clearly as possible. The game closing vs the game minimizing are two very different issues.
The issue with the game minimizing in fullscreen is usually due to other applications. I've heard this can be caused by other applications like Cisco Webex, or League of Legends, and probably even more. You should try to close all other applications and try again. If that doesn't work, can you maybe try deleting your Documents/BattleForge/config.json to reset all your graphics settings? That way the game should be windowed again.